Comparing the Stability of Chairs

PROBLEMS For each chair conformation shown below, do a chair flip and draw the other chair conformation. 

So you can see that it is important to understand what makes chair conformations unstable. There is really only one rule you need to worry about a chair will be more stable with a group in an equatorial position, because it is not bumping into anything (this bumping is called steric hinderanee). Axial positions are bumping into other axial positions, but equatorial positions are not  

So what happens if you also have a chlorine atom on the ring that is axial while the tert-butyl group is equatorial  

This will essentially lock the ring in the chair conformation that puts the chlorine in an axial position. If we are trying to am a reaction where the Cl needs to be axial, then this effect will speed up the reaction. However, if the Cl is locked in an equatorial position, then the reaction will be too slow  

Now we understand why this can be important. So let s go step by step in determining which of two chair conformations is more stable. 

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Let s compare the stability of chair conformations once again, this time for compounds that bear more than one substituent. Consider the following example ... 

Problem 9.16 (a) Draw the possible chair conformational structures for the following pairs of dimethylcy-dohexanes (i) cis- and trans-1,2-, (ii) cis- and frans-1,3- (iii) cis- and trans-lA-. (b) Compare the stabilities of he more stable conformers for each pair of geometric isomers, (c) Determine which of the isomers of limethylcyclohexane are chiral. <... 

Every compound with a cyclohexane ring has two chair conformers thus, both the cis isomer and the trans isomer of a disubstituted cyclohexane have two chair conformers. Let s compare the structures of the two chair conformers of cw-l,4-dimethylcyclohexane to see if we can predict any difference in their stabilities. 

The substitution of a heteroatom for an a-sulfoxy methylene group substantially increases the preference for an axial orientation of the sulfoxide oxygen320, despite the smaller space requirement of the sulfur with its lone pairs, compared to that of a methylene group321, at least in the case of 1,3-dithiolane oxides. The substituting heteroatom, therefore, should decrease the conformation stability (i.e. lower the barrier to chair-chair interconversion). 

For the cyclization of 10, there are four diastereomeric chair-like transition states, 22, 24, 25, and 26 (Tab. 16.1), each leading to one of the four possible diastereomeric products. The minimization of the four transition states with Mechanics [14], using the angles and bonds as stated, demonstrated transition state 22 to be the lowest in energy, being 5.3 kcal moh lower than the next most stable TS. This contrasts with the relative stability of the four diastereomeric products, 11, 12, 13, and 14 (Tab. 16.1), which are quite comparable one to another. 

Transannular cation radicals with the intramolecular sulfur-sulfur bond of the 2ct-1ct type generated from medium-ring disulfides like 1,5-dithiacyclooctane are an exception in terms of their stability, although they are not resistant to water (Musker 1980). ESR and resonance Raman spectroscopy studies revealed the existence of the 1,5-dithia-cyclooctane cation radical, with substantial bonding between the sulfur atoms (T. Brown et al. 1981 Tamaoki et al. 1989). Computations confirmed this statement and pointed out that the chair-boat conformer has the lowest energy as compared to other possible conformers (Stowasser et al. 1999). 

Problem 9.7 Compare stabilities, of. the possible chair conformations of (a) cis ... 

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